1. Field of the Invention
The present invention relates to an electrolytic in-process dressing method, an electrolytic in-process dressing grinding apparatus, and an electrolytic in-process dressing grindstone, for grinding workpiece as the grindstone is being subjected to electrolytic in-process dressing.
2. Discussion of Background
Recently metal bonded grindstones comprising abrasive grains with high degree of hardness have been developed as being useful for grinding hard and tough materials such as ceramics with high efficiency and as a matter of fact, the advantageous effects thereof have been appreciated.
Such grindstones, however, have the shortcomings that the tooling performance and dressing performance thereof are poor, and in-process dressing is difficult, so that stable machining is difficult to perform by use of such grindstones in conventional grinding techniques.
As a method of solving such conventional problems, electrolytic in-process dressing grinding (hereinafter referred to as "ELID-grinding"), which is capable of promoting in-process dressing in the course of grinding, has been developed and is recently attracting attention.
An example of such an ELID-grinding method is disclosed, for example, in Japanese Laid-Open Patent Application 6-254754.
In the ELID-grinding method, there is employed a grindstone comprising a binding material composed of an iron-based metal as the main component and other metallic additives, or a binding material composed of at least two components selected from the group consisting of cast iron, cobalt, nickel and copper. In this method, the above binding material is used as a positive electrode, and a negative electrode is disposed with a predetermined distance from a contact surface of the grindstone, with an electroconductive fluid with low conductivity being placed between the positive electrode and the negative electrode in contact therewith, a voltage is applied across the positive electrode and the negative electrode, so that the binding material is electrolytically eluted.
More specifically, as illustrated in FIG. 14, when the above-mentioned binding material which serves as the matrix material for supporting abrasive grains (hereinafter referred to as grains) comes into contact with the above-mentioned electroconductive fluid which is, for instance, an aqueous liquid serving as grinding fluid, iron ions are eluted from the binding material, so that in the electrolytic predressing, grains 1 begin to protrude from the surface of the grindstone due to the elusion of the iron ions as illustrated in (A) in FIG. 16, and at the same time, the eluted iron ion is bonded to hydroxyl ion to form iron hydroxide or iron oxide, and the thus formed iron hydroxide or iron oxide accumulates on the surface of the grindstone, whereby a passive layer is formed as illustrated in (B) in FIG. 14.
When the thickness of the passive layer increases to a certain thickness, the electric resistivity of the grindstone increases and the current which flows the binding material decreases, so that the elusion of the iron ions from the binder material is hindered. Thus, the protrudent grains 1 can grind a workpiece (not shown) as illustrated in (C) in FIG. 14. When the protrudent grains 1 wear down in the course of the grinding of the workpiece to the same level as that of the passive layer, the passive layer is worn down or removed in contact with the workpiece as illustrated in (D) in FIG. 14.
When the thickness of the passive layer decreases to a certain thickness, the resistivity of the grindstone increases and the elusion of iron ions from the binding material begins, so that the redressing of the grindstone is carried out with the recovery of the previously mentioned passive layer as shown in (B) in FIG. 14, whereby the so-called ELID cycle is repeated. Thus, the protrusion of the grains remains constant in general use, and stable grinding can be performed constantly for an extended period of time.
Thus, in a grindstone comprising a binding material composed of an iron-based metal as the main component and other metallic additives, or in a grindstone comprising a binding material composed of at least two components selected from the group consisting of cast iron, cobalt, nickel and copper, the dressing of the grindstone can be performed during grinding operation, that is, electrolytic in-process dressing can be performed during grinding operation. Therefore, it is unnecessary to stop grinding operation from time to time during grinding operation due to the loading of the grindstone.
Furthermore, as mentioned previously, a conventional grindstone for use in ELID-grinding method comprises a binding material composed of an iron-based metal as the main component and other metallic additives, or a binding material composed of at least two components selected from the group consisting of cast iron, cobalt, nickel and copper, so that the grindstone is highly wear resistant to workpieces to be ground. Furthermore, a passive layer can be easily formed and the dressing of the grindstone can be performed by electrolytic in-process dressing.
However, the above-mentioned conventional grindstone has the problem that the ground surface obtained by the grindstone is rougher than those obtained by grindstones comprising other binding materials, for instance, a vitrified grinding wheel. In particular, when obtaining a mirror surface in a hard and brittle material, scratches and cracks are apt to be formed by the above-mentioned conventional grindstone, so that the quality of the ground product is lowered.
Furthermore, when any of the above-mentioned conventional grindstones is employed in ELID-grinding method, a period of at least 10 minutes is required to perform predressing, so that the grinding time and efficiency are not always good.